Technical Field
The present invention relates to microelectronics arrangements, in particular to a photosensitive pixel array and to an implant with such a pixel array.
Description of the Related Art
Retinal implant systems are known, which are able to at least partially restore vision to patients who have lost sight, for instance through degenerative diseases such as retinitis pigmentosa. Vision may at least to a certain degree be restored with an implant by exploiting the fact that the degenerated tissue of the retina may be bypassed and that the remaining retina cells may be stimulated electrically. This electrical stimulation can be provided by means of an implant system. It is well known that neural tissue can be artificially stimulated and activated by implants that pass pulses of electrical current through electrodes to neuronal cells. The passage of current causes changes in electrical potentials across neuronal cell membranes, which can initiate neuronal action potentials, which are the means of information transfer in the nervous system. Based on this mechanism, it is possible to input information into the nervous system by coding sensory information input as a sequence of electrical pulses which are relayed to the nervous system via the implant system. In this way, it is possible to provide artificially generated sensations including vision. Such a system typically comprises a head-mounted arrangement (e.g. in the form of goggles), which is placed in front of an eye of a patient, and an implant, in particular a sub-retinal implant, which comprises a plurality of electrodes.
In those goggles, typically a camera is provided. The camera is adapted to capture the visual scene in front of the patient. This captured scene information is processed by a computer and converted into a related light pulse signal. The implant is adapted to receive those light pulses and, in response, to convert light into electrical current that stimulates the residual cells in the retina.
For that purpose, the implant comprises an array of electrically isolated pixels, wherein each pixel comprises one or more photosensitive elements such as photodiodes and a stimulating electrode. The photosensitive elements receive the light pulses and convert the information carried by the light input into electrical signals that are conveyed to the electrodes.
An example of retinal implant including photosensitive pixel array is described in Mathieson et al., “Photovoltaic retinal prosthesis with high pixel density”, Nat Photon, vol. 6, pp. 391-397, 2012. In such a system, the implant includes an array of pixel cells, each pixel cell implementing a microelectronics circuit with an optical receiver pixel array of photodiode elements for converting received pulsed near-IR (˜900 nm) light into biphasic pulses of electric current able to stimulate retinal neurons. Although an array of pixel cells can be limited to a single cell, it will generally include a number of pixel cell circuits corresponding to a desired spatial resolution for stimulation of the eye's tissue.
In order to improve performance of these implants including photosensitive pixel array, for example by increasing quality of retina stimulation, temporal or spatial resolution, visual acuity, etc., it has been proposed to increase the achievable light pulse repetition rate beyond state-of-the-art rates of about 30 Hz, possibly towards the native temporal resolution of retinal cells of the order of 1000 Hz or higher.
The light-sensitive (or photosensitive) elements in microelectronic implementations of state-of-the-art retinal implants are realized in the form of semiconductor photodiodes. The electrical circuit composed of the stimulation electrodes and the photodiodes entails (parasitic) capacitances that are alternately charged and discharged by currents flowing in and out these capacitances. The charging phase (when the light pulse turns ON) is dependent on the light-induced photocurrent whereas the discharge (light is OFF) depends on currents flowing through the high-impedance path via the tissue to a return electrode. Due to this impedance limiting discharge currents to values much smaller than the photocurrents, the discharge time can become long compared to the charging time.
An incomplete discharge at the time of another stimulation light pulse arriving at the same implant pixel decreases the achievable charge injection of this second stimulation pulse and therefore limits the stimulation efficiency of the implant pixel. As a consequence, the repetition rate of stimulation light pulses sent to one implant pixel becomes limited if a certain minimum stimulation efficiency is to be retained, impacting the achievable temporal resolution of stimulation. (see Loudin et al., 2011, IEEE Transaction on Biomedical Circuits and Systems, 5, 468-480). It appears that actually, the photovoltaic current (and thereby stimulation efficiency) decreases exponentially with increasing light pulses frequency (Loudin et al., 2011, supra). To speed-up discharge of the electrode between the light pulses and thereby avoid charge accumulation and the associated decrease of current with consecutive pulses, it has been proposed to add a shunt resistor. For example, Wang et al. (2012, J. Neural Eng., 9, 1-11) describe that the addition of a shunt resistor will help to speed up the discharge phase of the stimulation waveform or Loudin et al. (2011, IEEE Transaction on Biomedical Circuits and Systems, 5, 468-480) suggest the use of a shunt resistor for photodiode circuits in retinal prostheses. The shunt resistor allows the charge delivered during the first phase of the light pulse to be more rapidly discharged but also impacts the charge, which is actually delivered to the tissue. It has been shown for example that if resistance of the shunt resistor is too high, the electrodes will not completely discharge between the pulses and that the accumulation of charge on the electrode will reduce the amount of charge delivered during the successive pulses. As a result, there exists a trade-off between achievable stimulation efficiency and temporal resolution of stimulation as long as a fixed-value resistor is used as the shunt device. As a consequence, in state-of-the-art implementations, the shunt resistor value is optimized for a certain compromise between these two conflicting parameters. (Boinagrov et al., Photovoltaic Pixels for Neural Stimulation: Circuit Models and Performance, January 2015, IEEE Transactions on Biomedical Circuits and System).
Therefore there is still a need for providing an improved pixel cell circuit and implants, such as photovoltaic visual implants, incorporating the same that address the above-described drawbacks and allows stimulation with light pulses rates beyond about 30 Hz, more specifically beyond about 50 Hz, and even more specifically beyond about 100 Hz without decreasing electrode stimulation efficiency. In other words, there is a need for microelectronics arrangements that will break the detrimental interdependence between achievable stimulation pulse repetition rate and stimulation efficiency.